Superconductivity in few-layer iron chalcogenide and transition metal dichalcogenide devices

: (Alternative Format Thesis)

Abstract

This thesis presents a study of superconducting phenomena in iron chalcogenides (FeCs) and transition metal dichalogenides (TMDs). Using mechanical exfoliation and dry transfer methods, thin flake devices have been fabricated from these materials. These devices were characterised primarily using magnetotransport measurements to examine changes in the superconducting critical temperature, electronic structure and carrier dynamics.

The iron chalcogenides are ideal candidates for studying the mechanism of unconventional pairing in iron-based superconductors due to their relatively simple crystal structure, and the ready availability of high-quality bulk single crystals. Furthermore, due to the weak van der Waals bonding of the C-Fe-C layers, these materials cleave readily, allowing the exploration of unconventional superconductivity in a perfectly crystalline sample that can be thinned down to the single layer level. Additionally, there is significant interest in both understanding and characterising the physics associated with van der Waals-bonded superconductors, as well as the incorporation of them into current and next-generation technologies.

The bulk of this thesis concerns an experimental study of exfoliated iron selenide (FeSe) samples as they are thinned down towards the monolayer limit. Measurements of the superconducting critical temperature and upper critical field reveal that as the thickness of FeSe is reduced below ~20 nm, there is a transition from a weakly anisotropic superconducting regime, to a highly anisotropic regime with strong two-dimensional character where superconductivity is suppressed. Using a two band model and mobility spectrum analysis, it was found that there is a dichotomy between the behaviour of electrons and holes in thin flakes. While hole mobilities remain high at low temperatures, electron mobilities are observed to fall substantially, suggesting that these carriers become localised. This is in stark contrast to what is observed in bulk FeSe, where a very high mobility electron pocket emerges below 10K.

The final chapter concerns incorporating 2H-NbSe₂ and, by a wider extension, other 2D materials into Josephson junction (JJ) and superconducting quantum interference device (SQUID) structures. Utilising a dry transfer technique, it was found that NbSe₂ can be stacked to form a JJ in which the junction dynamics can be tuned by varying the misalignment angle between the flakes. Using a relatively simple fabrication procedure, these junctions have been etched into SQUID geometries. The resulting NbSe₂ SQUIDs display large stable current and voltage modulation depths in an applied magnetic field.

Date of Award	28 Apr 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Simon Crampin (Supervisor) & Simon Bending (Supervisor)